One of the major problems associated with using the low rank coals such as subbituminous or lignites as found in the Western United States is their high moisture content, usually ranging from 15 to 50 wt. %. If such coal is shipped to the consumer, there is a large weight penalty due to the negative value of the water content. Also, upon burning the coal, considerable heat is required to vaporize the high moisture content which reduces overall process efficiency.
It has been proposed that such low rank subbituminous coals or lignites be dried at the mine before shipment or use. However, it has been observed that when dried, the coal undergoes substantial reabsorption of moisture and consequential heating, which makes the coal subject to spontaneous ignition during shipment and/or subsequent storage. Furthermore, the desirability of passivating such dried coals by coating them with a heavy oil or tar material so as to substantially prevent the reabsorption of moisture has been generally recognized.
For example, the U.S. Pat. No. 1,905,513 to Stuart describes a method for filming (coating) coal with a preserving hydrocarbon film which is impervious to both air and water so as to help prevent dusting and oxidation of the coal before burning. U.S. Pat. No. 1,960,917 to Nagelvoort describes a method for removing excess moisture from wetted coal by spraying it with a dilute oil emulsion to facilitate the drainage of excess water from the coal. Also, U.S. Pat. No. 2,197,792 to Erickson describes apparibes apparatus for spraying of coal with oil or wax to prevent dusting, while Wattles -- U.S. Pat. No. 2,204,781 -- describes coating exposed surfaces of coal piles with a protective weather-excluding coating material. Furthermore, Lykken -- U.S. Pat. Nos. 2,610,115 and 2,811,427 -- describe a method for dehydrating lignite by mixing it with 3 to 10% mineral hydrocarbon at normal temperature and then heating the mixture to about 300.degree. F in a rotating kiln to remove moisture and leave the lignite particles coated with the hydrocarbon material. Although the problem has been recognized for more than 50 years, and there are some reported laboratory scale data, there has been, to date, no practical process for accomplishing such passivation of wet coals and lignites on a commercial scale which will provide such a substantially moisture-free, stable, low rank coal.